Display device and manufacturing method thereof

ABSTRACT

The purpose of the invention is to form a stable oxide semiconductor TFT in a display device. The concrete structure is: A display device having a TFT substrate that includes a TFT having an oxide semiconductor layer comprising: the oxide semiconductor layer is formed on a first insulating film that is formed by a silicon oxide layer, the oxide semiconductor layer and an aluminum oxide film are directly formed on the first insulating film. The first insulating film becomes oxygen rich when the aluminum oxide film is formed on the first insulating film by sputtering. Oxygens in the first insulating film is effectively confined in the first insulating film, eventually, the oxygens diffuse to the oxide semiconductor for a stable operation of the oxide semiconductor TFT.

CLAIM OF PRIORITY

The present application is a continuation of U.S. application Ser. No.16/051,532, filed Aug. 1, 2018, which claims priority from JapanesePatent Application JP 2017-166818 filed on Aug. 31, 2017, the content ofwhich is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The present invention relates to a display device having an oxidesemiconductor TFT characterized in that a leak current due todeoxidization of the oxide semiconductor is suppressed.

(2) Description of the Related Art

A liquid crystal display device has a TFT substrate, a counter substrateopposing to the TFT substrate and a liquid crystal layer sandwichedbetween the TFT substrate and the counter substrate. The TFT substratehas plural pixels; each of the pixels has a pixel electrode and a thinfilm transistor (TFT). A transmittance of light in each of the pixels iscontrolled by liquid crystal molecules; thus, images are formed. On theother hand, an organic EL display device has a self-illuminant organicEL layer and a TFT in each of the pixels, thus, color images are formed.The organic EL display device does not need the back light, thus, it isadvantageous in forming a flexible display device.

In a display device, the TFTs are used for switching elements in thepixels, or used in the peripheral driving circuit. Since the TFT of theoxide semiconductor has a high OFF resistance, it is suitable for aswitching transistor. Furthermore, the TFT of the oxide semiconductorhas an advantage that it can be manufactured relatively in lowtemperature compared with the TFT of poly-Si.

Several insulating films are used as interlayer insulating films in thedisplay device. Many of them are a silicon oxide (herein after called bySiO in this specification) film or a silicon nitride (herein aftercalled by SiN in this specification); sometimes, an aluminum oxide filmis also used. The patent document 1 (Japanese patent application laidopen No. Hei 9-213968) discloses to form the gate electrode by aluminum;the surface of the aluminum is transformed to the aluminum oxide byanode oxidization; thus adherence between the gate electrode and theresist is improved. The patent document 1 further discloses that whenthe through hole is formed, the aluminum oxide in the through hole isremoved by etching.

SUMMARY OF THE INVENTION

In the TFT of the oxide semiconductor, a leak current increases when theresistance of the oxide semiconductor at the channel decreases due todeoxidization of the oxide semiconductor. The deoxidization of the oxidesemiconductor occurs when oxygens are extracted by metal and the like,or hydrogens are supplied to the oxide semiconductor from the layerformed by SiN, etc.

One method to prevent the oxide semiconductor from being reduced is todispose the SiO film in contact with the oxide semiconductor; and tosupply oxygens to the oxide semiconductor from the SiO film. The SiOfilm must be oxygen rich in order to supply oxygens to the oxidesemiconductor. The oxygen rich structure, however, contains a lot ofdefects; thus, the SiO film cannot maintain enough insulatingcharacteristics. Furthermore, even the SiO film contains a lot ofoxygens, the oxygens may escape to other portions; in that case, enoughoxygens may not be supplied to the oxide semiconductor.

The purpose of the present invention is to realize the structure thatthe oxygen rich SiO film is disposed in contact with the oxidesemiconductor to supply enough oxygens from the SiO film to the oxidesemiconductor; consequently, to realize the oxide semiconductor TFT thathas low leak current at the same time.

The present invention overcomes the above explained problem; theconcrete structures are as follows.

A display device having a TFT substrate that includes a TFT having anoxide semiconductor layer comprising: the oxide semiconductor layer isformed on a first insulating film that is formed by a silicon oxidelayer; the oxide semiconductor layer and an aluminum oxide film aredirectly formed on the first insulating film. In other words, oxygens inthe first insulating film are effectively confined in the firstinsulating film by the aluminum oxide film; and consequently, oxygens inthe first insulating film are efficiently supplied to the oxidesemiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the liquid crystal display device;

FIG. 2 is a cross sectional view of the display area of the liquidcrystal display device;

FIG. 3 is a plan view of the oxide semiconductor TFT;

FIG. 4 is a cross sectional view of FIG. 3 along the line A-A;

FIG. 5 is a cross sectional view that the first gate electrode isformed;

FIG. 6 is a cross sectional view that the first gate insulating film isformed;

FIG. 7 is a cross sectional view that the oxide semiconductor layer isdeposited;

FIG. 8 is a cross sectional view that the oxide semiconductor layer ispatterned;

FIG. 9 is a cross sectional view that the aluminum oxide film isdeposited;

FIG. 10 is a cross sectional view that the photo resist is removed;

FIG. 11 is a plan view that the aluminum oxide film is formed on the TFTsubstrate;

FIG. 12 is a plan view of the display area that the aluminum oxide filmis patterned;

FIG. 13 is a cross sectional view of the first example of the embodiment2;

FIG. 14 is a cross sectional view of the second example of theembodiment 2;

FIG. 15 is a cross sectional view of the embodiment 3;

FIG. 16 is a cross sectional view of the embodiment 4;

FIG. 17 is a plan view of the organic EL display device;

FIG. 18 is a cross sectional view of the display area of the organic ELdisplay device, which the present invention is applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail referring to thefollowing embodiments. In the explanation below, the invention is mainlyexplained in an example of the liquid crystal display device; however,the present invention is applicable to the organic EL display device,too.

Embodiment 1

FIG. 1 is a plan view of a liquid crystal display device, which thepresent invention is applied. In FIG. 1, the TFT substrate 100, on whichthe TFTs and pixel electrodes are formed, and the counter substrate 100adhere to each other at their periphery by sealing material 21; theliquid crystal is encapsulated between the TFT substrate 100 and thecounter substrate 200. The display area 20 is formed in the areasurrounded by the sealing material 21.

In FIG. 1, the TFT substrate 100 is made bigger than the countersubstrate 200; the area of the TFT substrate 100 where the countersubstrate 200 does not overlap is the terminal area 30; the driver IC 31is installed on the terminal area 30. The flexible wiring circuitsubstrate 32, which supplies powers or signals to the liquid crystaldisplay device, connects to the terminal 30.

In the display area 20 in FIG. 1, the scanning lines 11 extend in thelateral direction (x direction) and are arranged in the longitudinaldirection (y direction). The video signal lines 12 extend in thelongitudinal direction (y direction) and are arranged in the lateraldirection (x direction). Video signals are sent to the video signallines 12 from the driver IC 31, which is installed on the terminal area30. The pixel 13 is formed in the area surrounded by the scanning lines11 and the video signal lines 12.

In the liquid crystal display device of FIG. 1 uses a TFT, which isconstituted by the oxide semiconductor. The oxide semiconductor has anadvantage of low leak current. Therefore, it is suitable for a switchingelement in the pixel in the display area. On the other hand, the TFTconstituted by the poly silicon has larger leak current, however, thepoly silicon has a high mobility, thus, the poly silicon is used oftenfor a TFT in the peripheral driving circuit.

FIG. 2 is a cross sectional view of the display area 20 of the liquidcrystal display device in FIG. 1. The oxide semiconductor layer 104 isused in the TFT in FIG. 2. The TFT of the oxide semiconductor can makethe leak current low. By the way, the oxide semiconductors that areoptically transparent and amorphous are called TAOS (TransparentAmorphous Oxide Semiconductor). The examples of TAOS are IGZO (IndiumGallium Zinc Oxide), ITZO (Indium Tin Zinc Oxide), ZnON (Zinc OxideNitride), IGO (Indium Gallium Oxide), and so on. In the presentinvention, IGZO is used as the oxide semiconductor. The oxidesemiconductor may be called as TAOS in this specification.

In FIG. 2, the undercoat 101 is formed on the TFT substrate 100, whichis formed either by glass or resin. If the liquid crystal display deviceis intended to be a flexible display device, the TFT substrate 100 isformed by resin as e.g. polyimide. Glass also can be flexibly bendableif its thickness becomes 0.2 mm or less.

In many cases, the undercoat 101 is made of a laminated film of the SiNfilm and the SiO film. The laminated film can be a two layer structureof the SiN film and the SiO film or the structure that the SiN film issandwiched by the SiO films. The role of the SiO film is to blockimpurities from the glass substrate and the like. Another role of theSiO film is to improve adhesive strength between the TFT substrate 100and films formed on the TFT substrate 100. The SiN film blocks impurity,especially moisture, from the glass substrate 100 and the like.

The TFT is formed on the undercoat 101; the TFT in FIG. 2 is a so calleddual gate type TFT. Namely, a first gate electrode 102 is formed underthe semiconductor layer 104 and the second gate electrode 106 is formedabove the semiconductor layer 104. The first gate electrode 102 isformed on the same layer as the scanning line 11 depicted in FIG. 1, orconnects with the scanning line 11 via a through hole. The first gateelectrode 102 is made of e.g. MoW alloy. If the electrical resistance ofthe first gate electrode 102 or the scanning line 11 is required to below, the structure that an aluminum layer is sandwiched by titaniumlayers and the like is adopted. In the meantime, if the TFT is intendedto be a top gate type, not a dual gate type, the first gate electrode102 is not connected to the scanning line 11, and can work as a lightstopping film for the oxide semiconductor layer 104.

The first gate insulating film 103 is formed on the first gate electrode102. The first gate insulating film 103 is formed by an oxygen rich SiOlayer, which supplies oxygens to the oxide semiconductor layer 104formed on the first gate insulating film 103. The present invention hasa characteristic in that oxygens are efficiently supplied to the oxidesemiconductor layer 104 from the first gate insulating film 103

In FIG. 2, the oxide semiconductor layer 104, which constitutes the TFT,is formed on the first gate insulating film 103 by sputtering or thelike. A thickness of the oxide semiconductor layer 104 is 5 nm to 100nm. The oxide semiconductor layer 104 is formed all over the substrate,subsequently, it is patterned.

In FIG. 2, the aluminum oxide film (herein after AlO film in thisspecification) 50 is formed on the first gate insulating film 103separated by a small distance d1 from the oxide semiconductor layer 104.The AlO film 50 is formed on all over the display area 20 except thearea of the oxide semiconductor layer 104 is formed. AlO film 50 isformed by sputtering under the oxygen rich atmosphere. Therefore, agreat amount of oxygens are implanted into the first gate insulatingfilm 103 during the AlO film 50 is being formed by sputtering.

The oxygens in the first insulating film 103 are enclosed in the firstgate insulating film 103 by the AlO film 50 after the AlO film 50 isformed by sputtering on the first gate insulating film 103. The enclosedoxygens in the first gate insulating film 103 diffuse to the side of theoxide semiconductor 104; thus, the oxide semiconductor 104 is suppliedwith oxygens. Consequently, deoxidization of the oxide semiconductor 104is avoided; as a result, the characteristics of the oxide semiconductorcan be maintained stable. The distance d1 between the oxidesemiconductor 104 and the AlO film 50 in FIG. 2 is due to a requirementof patterning accuracy of the AlO film 50 as explained later, thus, thedistance d1 can be 10 microns or less, and preferably 1 micron or less;and the value of d1 is the smaller the better.

In FIG. 2, the second gate insulating film 105 is formed on the oxidesemiconductor layer 104. The second gate insulating film 105, which ismade of SiO, is formed all over the display area; then patterned toremain only on the channel region of the oxide semiconductor layer 104.The second gate electrode 106 is formed on the second gate insulatingfilm 105. The second gate electrode 106 is either formed on the samelayer as the scanning line 11 or connected with the scanning line 11 viaa through hole. The second gate electrode 106, as the same as the firstgate electrode 102, is made of e.g. MoW alloy; if the electricalresistance is required to be low, the structure that an aluminum (Al)layer is sandwiched by titanium (Ti) layers and the like is adopted.

In the present invention, since the first gate insulating film 103 bearsa role to supply oxygens to the oxide semiconductor layer 104, thesecond gate insulating film 105 can be formed in focusing thecharacteristics as the gate insulating film. Namely, the second gateinsulating film 105 is not necessarily formed as oxygen rich. If oxygensare rich in the SiO layer, insulating characteristics of the SiO film isdeteriorated. Consequently, since a thickness of the second gateinsulating film 105 can be thin in the present invention, the functionof the TFT is more influenced by the top gate (second gate electrode)106.

On the other hand, in the structure of FIG. 2, the first gate insulatingfilm 103 is oxygen rich; the oxygens are supplied continuously from thearea of the first gate insulating film 103 that is covered by the AlOfilm 50. Therefore, it is not necessary to make only the portion in thefirst gate insulating film 103 that contacts with the oxidesemiconductor layer 104, extremely oxygen rich to supply oxygens to theoxide semiconductor layer 104; thus, the characteristics of the oxidesemiconductor layer 104 can be maintained stable.

After the second gate electrode 106 is formed, and before the interlayerinsulating film 107 is formed, the plasma treatment is applied to thesurface of the oxide semiconductor layer 104 to deoxidize the oxidesemiconductor layer 104 to give conductivity except the portion that theoxide semiconductor layer 104 is covered by the second gate insulatingfilm 105. Consequently, the drain region and the source region areformed in the oxide semiconductor 104. After that, the interlayerinsulating film 107 is formed to cover the second gate electrode 106,the oxide semiconductor layer 104, the AlO film 50, and so on. Theinterlayer insulating film 107 is made of SiO. If the interlayerinsulating film 107 is made of two layer structure of the SiO film andthe SiN film, the SiO film is disposed in the lower layer, which isnearer to the oxide semiconductor layer 104.

After that, through hole 120 and through hole 121 are formed in theinterlayer insulating film 107. The drain electrode 108 is formed at thethrough hole 120, and the source electrode 109 is formed at the throughhole 121. The drain electrode 108 connects with the video signal line12, and the source electrode 109 connects with the pixel electrode 113.

The organic passivation film 110 is formed to cover the drain electrode108, the source electrode 109 and the interlayer insulating film 107.The organic passivation film 110 is formed by photosensitive resin madeof e.g. acrylic and the like. Since the organic passivation film 110 hasa role as a flattening film, it is made as thick as 2 to 4 microns. Thethrough hole 130 is formed in the organic passivation film 110 toconnect the source electrode 109 and the pixel electrode 113.

In FIG. 2, the common electrode 111, which is made of the transparentconductive metal oxide film e.g. ITO (Indium Tin Oxide) film and thelike is formed on the organic passivation film 110. The common electrode111 is formed in common to the plural pixels. The capacitive insulatingfilm 112 is formed to cover the common electrode 111; the pixelelectrode 113 is formed on the capacitive insulating film 112. Since astorage capacitance is formed between the pixel electrode 113 and thecommon electrode 111 sandwiching the capacitive insulating film 112, theinsulating film 112 between the pixel electrode 113 and the commonelectrode 111 is called the capacitive insulating film 112.

The alignment film 114, which is for initial alignment of the liquidcrystal molecules 301, is formed to cover the pixel electrode 113 andthe capacitive insulating film 112. The alignment film 114 getsalignment ability through either by rubbing process or by photoalignment process using polarized ultra violet rays. The photo alignmentprocess has advantage in IPS (In Plane Switching) type liquid crystaldisplay device.

A through hole is formed in the capacitive insulating film 112 in thethrough hole 130 in the organic passivation film 110 to make contactbetween the pixel electrode 113 and the source electrode 109. The pixelelectrode 113 is formed as comb shaped or stripe shaped; when videosignal is applied to the pixel electrode 113, the electrical line offorce as depicted in FIG. 2 is generated between the pixel electrode 113and the common electrode 111 to rotate the liquid crystal molecules 301;thus, a transmittance of the liquid crystal layer 300 is controlled.

In FIG. 2, the counter substrate 200 is disposed to sandwich the liquidcrystal layer 300 between the TFT substrate 100. The counter substrate200 also can be made of glass or resin, the same as the TFT substrate100. The color filter 201 and the black matrix 202 are formed inside ofthe counter substrate 200. The color filter 201 is formed at the placecorresponding to the pixel electrode 113 in a plan view, thus, colorimages are formed. The black matrix 202 is formed at the placecorresponding to the through hole 130 and the TFT, thus, leak of lightfrom the backlight is prevented.

The overcoat film 203 is formed to cover the color filter 201 and theblack matrix 202. The overcoat film 203 prevents the liquid crystallayer 300 from being contaminated by pigments of the color filter 201.The alignment film 204 is formed over the overcoat film 203 for initialalignment of the liquid crystal molecules 301. It is the same as thealignment film 114 on the TFT substrate 100 in that the alignment film204 gets alignment ability through either by rubbing process or by photoalignment process using polarized ultra violet rays.

FIG. 3 is a schematic plan view of the TFT in FIG. 2. In FIG. 3, thefirst gate electrode 102 is formed as the lowest layer. The first gateelectrode 102 is connected to the scanning line 11 via the through holeat a place not shown in FIG. 2. The oxide semiconductor layer 104 andthe AlO film 50 are formed over the first gate electrode 102 through thefirst gate insulating film 103. The oxide semiconductor layer 104constitutes the TFT; the AlO film 50 has a role to confine oxygens inthe first gate insulating film 103. The AlO film 50 is formed all overthe display area 20 except the place where the oxide semiconductor layer104 is formed. The distance d1 between the oxide semiconductor layer 104and the AlO film 50 is determined by process requirement; the distanced1 is the smaller the better; concretely, 10 microns or less,preferably, 1 micron or less.

In FIG. 3, the second gate electrode 106 is formed over the oxidesemiconductor layer 104 through the second gate insulating film 105. InFIG. 3, the second gate electrode 106 is formed on the same layer as thescanning line 11. The second gate electrode 106 is covered by theinterlayer insulating film 107; the through hole 120 and through hole121 are formed in the interlayer insulating film 107. The drainelectrode 108 is formed to cover the through hole 120; the sourceelectrode 109 is formed to cover the through hole 121.

FIG. 4 is a cross sectional view of FIG. 3 along the line A-A. FIG. 4 isan enlarged cross sectional view of the TFT portion; the undercoat 101is not shown in FIG. 4. In FIG. 4, the first gate electrode 102 isformed on the TFT substrate 100; the first gate insulating film 103 isformed to cover the first gate electrode 102. The oxide semiconductorlayer 104 is formed over the first gate electrode 102 through the firstgate insulating film 103. The AlO film 50 is formed on the first gateinsulating film 103 with a slight distance d1 to the oxide semiconductorlayer 104. The AlO film 50 is formed all over the upper surface of thefirst gate insulating film 103 except the place where the oxidesemiconductor layer 104 is formed.

Oxygens are implanted in the first gate insulating film 103 when the AlOfilm 50 is formed by sputtering; the oxygens are confined in the firstgate insulating film 103 by the AlO film 50. The second gate insulatingfilm 105 is formed on the oxide semiconductor layer 104; the second gateelectrode 106 is formed on the second gate insulating film 105. Theoxide semiconductor layer 104 is deoxidized by the plasma treatment toget conductivity except the region that the second gate electrode 106and the second gate insulating film 105 are formed. The structures ofthe interlayer insulating film 107, drain electrode 108, the sourceelectrode 109, and the like are the same as explained in FIG. 2.

In FIG. 4, oxygens confined in the first gate insulating film 103diffuses to the oxide semiconductor layer 104 to prevent the oxidesemiconductor layer 104 from being deoxidized; thus, the variation ofcharacteristics of the oxide semiconductor layer 104 can be avoided fora long time. In the structure of FIG. 4, the first gate insulating film103 is oxygen rich; in this case, oxygens are continuously supplied tothe oxide semiconductor layer 104 from the area that the first gateinsulating film 103 is covered by the AlO film 50. Therefore, for thepurpose of stably supplying oxygens to the oxide semiconductor layer104, it is not necessary to make extremely oxygen rich only in theregion that the first gate insulating film 103 contacts the oxidesemiconductor layer 104. Thus, the characteristics of the semiconductorlayer 104 can be maintained in stable.

On the other hand, the second gate insulating film 105, which is formedon the oxide semiconductor layer 104, is not necessarily a source ofoxygens for the oxide semiconductor layer 104; thus, the second gateinsulating film 105 can be formed focused in having a good insulatingcharacteristic. Therefore, a thickness of the second gate insulatingfilm 105 can be thin.

FIGS. 5 through 10 are the interim cross sectional views in theprocesses to realize the structure of FIG. 4. In FIGS. 5 through 10, theundercoat is not shown. FIG. 5 is a cross sectional view that the firstgate electrode 102 is formed on the TFT substrate 100. FIG. 6 is a crosssectional view that the first gate insulating film 103 is formed tocover the first gate electrode 102 and the TFT substrate 100. The firstgate insulating film 103 is made of SiO.

After that as depicted in FIG. 7, the oxide semiconductor layer 104 isformed on the first gate insulating film 103. The oxide semiconductorlayer 104 is formed by e.g. sputtering in thicknesses of 5 nm to 100 nm.FIG. 8 is a cross sectional view that the oxide semiconductor layer 104is patterned with the photo resist 500. The oxide semiconductor layer104 is patterned by wet etching; at this time, the side etching as shownby SE is generated.

FIG. 9 is a cross sectional view that the AlO film 50 is formed bysputtering in thicknesses of 1 nm to 50 nm on the structure of FIG. 8,and formed on all over the display area in the same manner. That meansthe AlO film 50 is formed on the on the resist 500, too. During the AlOfilm 50 is formed by sputtering, a great amount of oxygens are implantedin the first gate insulating film 103 except the portion that is coveredby the resist 500. After that, when the resist 500 is removed asdepicted in FIG. 10, the AlO film 50, which is formed on the resist 500,is also removed.

During the sputtering of the AlO film 50, a thickness of the AlO film 50tends to be locally thin under the edge of the resist, even though theshape is not stable, because of the side etching SE of the oxidesemiconductor layer 104; consequently, a void tends to be formed at thisposition. Therefore, an edge of the AlO film 50 tends to be reversedtaper.

In FIG. 10, the gap d1, corresponding to the side etch SE formed whenthe oxide semiconductor layer 104 is patterned by etching, is formedbetween the oxide semiconductor layer 104 and the AlO film 50. If d1 isbig, oxygens that diffuse in the first gate insulating film 103 escapefrom the gap d1 to the outside; consequently, the oxygens that reach theoxide semiconductor layer 104 decrease. Thus, the gap d1 is smaller thebetter if the process allows. The gap d1 is 10 microns or less, andpreferably 1 micron or less if the process allows. After that, thesecond gate insulating film 105, the second gate electrode 106, theinterlayer insulating film 107, the through hole 120, 121, the drainelectrode 108 and the source electrode 109 are formed to complete thestructure of FIG. 4.

FIG. 11 is a plan view that the AlO film 50 is formed by sputtering onall over the TFT substrate 100. In FIG. 11, broken lines indicate theareas that the scanning line driving circuit area 60, the controlcircuit area 61 and the terminal area 30 are to be formed; however,those structures are not formed yet when the AlO film 50 is beingsputtered.

FIG. 12 is a plan view that shows the AlO film 50, which is formed allover the display area, is removed from the area that the oxidesemiconductor 104 is formed. In other words, the AlO film 50 covers allthe area except the place where the oxide semiconductor 104 is formed.

As described above, according to the present invention, the first gateinsulating film 103 can contain a great amount of oxygens in it when theAlO film 50 is formed by sputtering on the first gate insulating film103; further, the oxygens can be confined in the first gate insulatingfilm 103 by the sputtered AlO film 50, which is formed to cover thefirst gate insulating film 103. Further, the oxygens confined in thefirst gate insulating film 103 diffuse in the first gate insulating film103 to supply the oxygens to the oxide semiconductor layer 104; thus,variations of the characteristics of the oxide semiconductor layer 104can be efficiently suppressed.

Embodiment 2

FIG. 13 is a cross sectional view that shows the embodiment 2. FIG. 13differs from FIG. 4 of the embodiment 1 in that no gap exists betweenthe oxide semiconductor layer 104 and the AlO film 50, but the edge ofthe AlO film 50 is laid over the edge of the oxide semiconductor layer104. If there is no gap between the AlO film 50 and the oxidesemiconductor layer 104, the oxygens in the first gate insulating film103 cannot escape through the gap between the AlO film 50 and the oxidesemiconductor layer 104; thus, oxygens are efficiently supplied to theoxide semiconductor layer 104.

The structure of FIG. 13 is formed as that: the oxide semiconductorlayer 104 is formed and patterned, then the resist 500 is removed; afterthat the AlO film 50 is formed by sputtering, then, the AlO film 50 ispatterned by photolithography. Namely, the number of times of thephotolithography is one time more than the structure of theembodiment 1. Other processes are the same as explained in theembodiment 1.

FIG. 14 is cross sectional view of another example of the embodiment 2.The structure of FIG. 14, too, features that there is no gap between theAlO film 50 and the oxide semiconductor 104; however, FIG. 14 differsfrom FIG. 13 in that the edge of the oxide semiconductor layer 104 islaid over the edge of the AlO film 50. In other words, forming andpatterning of the AlO film 50 are made first, then, forming andpatterning of the oxide semiconductor layer 104 are made. FIG. 14 is thesame as FIG. 13 in that the number of times of the photolithography isone time more than the structure of the embodiment 1. In addition, thestructure of FIG. 14 has the same effect as that of FIG. 13.

Embodiment 2

In the embodiment 1, the TFT is dual gate type. The present invention,however, can be also applied when the TFTs are top gate type or bottomgate type. The top gate type is conceived as that the first gateelectrode 102 in the embodiment 1 is not a gate electrode but is a lightshielding film in the embodiment 2. Therefore, the bottom gate type TFTis explained in this embodiment.

FIG. 15 is a cross sectional view that the present invention is appliedto the bottom gate type TFT. In FIG. 15, the undercoat is not shown. InFIG. 15, the first gate electrode 102 is formed on the TFT substrate100. The first gate insulating film 103 is formed to cover the firstgate electrode 102. The oxide semiconductor layer 104 is formed over thefirst gate electrode 102 through the first gate insulating film 103. TheAlO film 50 is formed on the first gate insulating film 103 except thearea where the oxide semiconductor layer 104 is formed. The oxygens inthe first gate insulating film 103 are encapsulated by the AlO film 50.

After that, the drain metal 1081 and the source metal 1091 are formed tocover the edges of the oxide semiconductor layer 104. The drain metal1081 and the source metal 1091 are formed by the same material as thedrain electrode 108 or the source electrode 109 e.g. made of e.g. thestructure that an aluminum film is sandwiched by titanium films. Theoxygens are extracted from the oxide semiconductor layer 104 at theportion where the drain metal 1081 and the source metal 1091 contacts;thus, the oxide semiconductor layer 104 is conductive at this portion.In FIG. 15, the drain metal 1081 and the source metal 1091 partiallyoverlap with the first gate electrode 102 in a plan view.

In FIG. 15, the second gate insulating film and the second gateelectrode don't exist over the oxide semiconductor layer 104. Namely,the TFT is made ON or OFF only by the first gate electrode 102. In FIG.15, the interlayer insulating film 107 is formed to cover the oxidesemiconductor layer 104, the drain metal 1081 and the source metal 1091;the through holes 120 and 121 are formed in the interlayer insulatingfilm 107; subsequently, the drain electrode 108 and the source electrode109 are formed in the through hole 120 and through hole 121respectively.

The merit of FIG. 15 is that; the through holes 120 and 121, which areformed in the interlayer insulating film 107, is formed on the drainmetal 1081 or the source metal 1091; thus, the drain region or thesource region of the oxide semiconductor layer 104 don't disappear whenthe through holes 120 and 121 are formed. On the contrary, in theembodiment 1, as depicted in FIG. 4, the through holes 120 and 121 areformed on the thin oxide semiconductor layer 104, thus, there is adanger that the oxide semiconductor layer 104 at the correspondingportion disappears according to the condition of etching. In this point,the structure of FIG. 15 has advantage for the production of the TFTwith high yield.

In this embodiment too, the oxygens in the first insulating film isefficiently confined in the first gate insulating film 103 by the AlOfilm 50; thus, the oxygens are effectively supplied to the oxidesemiconductor layer 104. Therefore, a reliability of the TFT constitutedby the oxide semiconductor layer 104 can be improved.

Embodiment 4

The present invention is applicable to so called hybrid type liquidcrystal display device, in which the TFT of the oxide semiconductor andthe TFT of the poly silicon are formed on the same TFT substrate 100.Since the oxide semiconductor 104 has a less leak current, it issuitable for the switching TFT in the pixel; since the poly silicon hashigh mobility, it is suitable for the TFT that is used in the drivercircuit, which is built in on the TFT substrate 100.

FIG. 16 is a cross sectional view that the TFT of the oxidesemiconductor 104 and the TFT of the poly silicon 140 are formed on thesame TFT substrate 100. In FIG. 16, the undercoat 101 is formed on theTFT substrate 100. The structure of the undercoat 101 is the same asexplained in FIG. 2. The semiconductor layer of poly-silicon 140 isformed on the undercoat 101. The poly silicon layer 140 is formed asthat: the a-Si (amorphous silicon) layer is formed by CVD, then the a-Silayer is transformed to the poly silicon layer 140 by applying excimerlaser on the a-Si layer. Such a poly silicon is called LTPS (LowTemperature Poly-Si). The poly silicon layer 140 of FIG. 16 is formed bypatterning the LIPS. In the meantime, some products use a-Si film, as itis, for the TFTs.

The third gate insulating film 141 is formed by SiO covering the LIPS140; the third gate electrode 142 is formed on the third gate insulatingfilm 141. The first gate electrode 102 for the TFT of the oxidesemiconductor 104 is formed simultaneously with the third gate electrode142. In the meantime, in some products, the first gate electrode 102 isused as a light shielding film, not the gate electrode. In that case,the gate voltage is not applied to the first gate electrode 102 so thatit works only to shield the light from the back light.

The first gate insulating film 103, made of SiO, is formed over thethird gate electrode 142 of the poly silicon TFT and the first gateelectrode 102 of the oxide semiconductor TFT. In the oxide semiconductorTFT side, the oxide semiconductor layer 104 is formed and patterned.After that, the AlO film 50 is formed on the first gate insulating film103 except the area the oxide semiconductor layer 104 is formed. Theprocess to form the AlO film 50 is the same as explained in FIG. 7 toFIG. 10.

As explained in the embodiment 1, oxygens are confined in the first gateinsulating film 103 by the AlO film 50; and the oxygens are efficientlysupplied to the oxide semiconductor layer 104. In the meantime, the AlOfilm 50 is also formed on the first gate insulating film 103 in the sideof the poly silicon TFT; however, this AlO film 50 does not have impactto the poly silicon TFT. In the poly silicon TFT side, through holes 122and 123 are formed; the drain electrode 115 and the source electrode 116are formed respectively in those through holes.

FIG. 16 is an example that the structure of the embodiment 1 is appliedto the hybrid structure on the TFT substrate 100; however, structures ofthe embodiment 2 and embodiment 3 are also applicable to the structureof the embodiment 4. As described above, the present invention,explained in the embodiment 1 and the other embodiments, can be appliedto the liquid crystal display device that has hybrid type TFTs on theTFT substrate; consequently, the oxide semiconductor TFTs of highreliability can be formed in the hybrid type structure.

Embodiment 5

The embodiment 1 through embodiment 4 explains the examples when thepresent invention is applied to the liquid crystal display device. Thepresent invention, however, can be applied to the organic EL displaydevice. FIG. 17 is a plan view of an example of the organic EL displaydevice. In FIG. 17, the organic EL display device has the display area20 and the terminal area 30. In the display area 20, the scanning lines11 extend in the lateral direction (x direction) and are arranged in thelongitudinal direction (y direction). The video signal lines 12 and thepower lines 14 extend in the longitudinal direction (y direction), andare arranged in the lateral direction (x direction). The power lines 14supply current to the organic EL layer in each of the pixels. The pixel13 is formed in the area surrounded by the scanning lines 11 and thevideo signal lines 12.

In FIG. 17, the scanning line driver circuits 60 are formed at bothsides of the display area 20; the current supply area 62 is formed atthe upper side (y direction) of the display area 20. The driver IC 31,which includes video signal line driver circuit, is installed on theterminal area 30. The flexible wiring circuit substrate 32, whichsupplies power and signals to the organic EL display device, connects tothe terminal 30.

FIG. 18 is a cross sectional view of the display area 20 of the organicEL display device. FIG. 18 is a cross sectional view at the place wherethe organic EL layer and the driving TFT to drive the organic EL layerare formed. In FIG. 18, the undercoat 101 is formed on the TFT substrate100; the first gate electrode 102 is formed on the undercoat 101; thefirst gate insulating film 103 is formed by SiO to cover the first gateelectrode 102. The oxide semiconductor layer 104 is formed over the gateelectrode 102 through the gate insulating film 103.

The AlO film 50 is formed on the gate insulating film 103 except theplace where the oxide semiconductor layer 104 is formed. As described inthe embodiment 1, the role of the AlO film 50 is to encapsulate oxygensin the oxygen rich first gate insulating film 103 so that the oxygensare efficiently supplied to the oxide semiconductor layer 104. Themanufacturing method of the AlO film 50 and a thickness of the AlO film50 and the like are the same as explained in the embodiment 1.

After that, the second gate insulating film 105 is formed on the oxidesemiconductor layer 104; the second gate electrode 106 is formed on thesecond gate insulating film 105. The interlayer insulating film 107 isformed to cover the oxide semiconductor layer 104 and the AlO film 50.The through holes 120 and 121 are formed in the interlayer insulatingfilm 107; the drain electrode 108 is formed in the through hole 120 andthe source electrode 109 is formed in the through hole 121. The organicpassivation film 110 is formed to cover the drain electrode 108 and thesource electrode 109; the through hole 130 is formed in the organicpassivation film 110 to connect the source electrode 109 and the anode402. As described above, up to the through hole 130 is formed in theorganic passivation film 110, the structure of the organic EL displaydevice is the same as the liquid crystal display device.

In FIG. 18, the reflection electrode 401 and the anode 402 are formed inlamination on the organic passivation film 110. The reflection electrode401 is made of e.g. a thin silver film; the anode 402 is made of ITO.The reflection electrode 401 and the anode 402 are also called lowerelectrode. The lower electrodes 401 and 402 connect with the sourceelectrode via the through hole 130. In the meantime, to improve theadhesion between the organic passivation film 110 and a lamination filmof the reflection electrode 401 and the anode 402, sometimes ITO isfurther formed under the reflection electrode 401.

The bank 403 is formed to cover the edges of the lower electrode 401,402. The role of the bank 403 is to partition the pixels, and to preventthe organic EL layer 404, which is formed on the lower electrode 401,402, from getting disconnection due to the step of the lower electrode401, 402. In FIG. 18, the organic EL layer 404 is formed on the anode402. The organic EL layer 404 comprises plural layers of the holeinjection layer, the hole transportation layer, the light emittinglayer, the electron transportation layer and the electron injectionlayer in an order from the anode 402.

In FIG. 18, the upper electrode 405, which is to be the cathode, isformed on the organic EL layer 404. The upper electrode 405 is made ofthe transparent conductive film like IZO (Indium Zinc Oxide), ITO(Indium Tin Oxide) and the like; or sometimes it is made of a thinmetal, like e.g. silver. Since the organic EL layer 404 is decomposed bymoisture, the protection layer 406 is formed by e.g. SiN covering theupper electrode 405 to prevent an intrusion of moisture. After that, thepolarizing plate 408 is attached on the protection layer 406 through theadhesive 407. The polarizing plate 408 is used to prevent the reflectionof external light.

FIG. 18 is an example that the structure of the embodiment 1 is appliedto the organic EL display device; however, the structures of embodiments2 through 4 are also applicable to the organic EL display device.

As described above, the structure of the oxide semiconductor TFT and thestructure vicinity to the oxide semiconductor TFT in the organic ELdisplay device can be made the same as that of the liquid crystaldisplay device. Therefore, oxygens in the SiO film under the oxidesemiconductor layer 104 can be encapsulated by the AlO film 50,consequently, oxygens are efficiently supplied to the oxidesemiconductor layer 104. Thus, the organic EL display device of lessvariation in characteristics can be realized.

What is claimed is:
 1. A display device comprising: a substrate; a firsttransistor with an oxide semiconductor layer above the substrate; afirst insulating film under the oxide semiconductor; and an aluminumoxide film above the substrate, wherein the oxide semiconductor layerand the aluminum oxide film are on and in contact with the firstinsulating film, and an end part of the aluminum oxide film overlies anend part of the oxide semiconductor layer, a first gate electrode facesthe oxide semiconductor layer, the first insulating film is locatedbetween the oxide semiconductor layer and the first gate electrode, andportion of the first gate electrode and the aluminum oxide film overlapin a plan view.
 2. The display device according to claim 1, wherein asecond insulating film is located on the oxide semiconductor layer, anda second gate electrode is located on the second insulating film.
 3. Thedisplay device according to claim 2, wherein the second insulating filmis located only on a channel of the oxide semiconductor layer.
 4. Thedisplay device according to claim 1, wherein a drain metal contacts afirst end part of the oxide semiconductor layer and does not contact thealuminum oxide film, and a source metal contacts a second end part ofthe oxide semiconductor layer and does not contact the aluminum oxidefilm, the second end part being at an opposite side of the first endpart.
 5. A display device comprising: a substrate; a first transistorwith an oxide semiconductor layer above the substrate; a firstinsulating film under the oxide semiconductor; and an aluminum oxidefilm above the substrate, wherein the oxide semiconductor layer and theoxide film are on and in contact with the first insulating film, and anend part of the aluminum oxide film overlies an end part of the oxidesemiconductor layer, a light shielding film faces the oxidesemiconductor layer and is located under the oxide semiconductor layerand the first insulating film, and no portion of the light shieldingfilm and the aluminum oxide film overlap in a plan view.
 6. The displaydevice according to claim 5, wherein a second insulating film is locatedon the oxide semiconductor layer, and a second gate electrode is locatedon the second insulating film.
 7. The display device according to claim6, wherein the second insulating film is located only on a channel ofthe oxide semiconductor layer.
 8. The display device according to claim5, wherein a drain metal contacts a first end part of the oxidesemiconductor layer and does not contact the aluminum oxide film, and asource metal contacts a second end part of the oxide semiconductor layerand does not contact the aluminum oxide film, the second end part beingat an opposite side of the first end part.